Communication systems exist which allow a live voice and/or video call to be conducted between two or more end-user terminals over a packet-based network such as the Internet, using a packet-based protocol such as internet protocol (IP). This type of communication is sometimes referred to as “voice over IP” (VoIP) or “video over IP”.
To use the communication system, each end user first installs a client application onto a memory of his or her user terminal such that the client application is arranged for execution on a processor of that terminal. To establish a call, one user (the caller) indicates a username of at least one other user (the callee) to the client application. When executed the client application can then control its respective terminal to access a database mapping usernames to IP addresses, and thus uses the indicated username to look up the IP address of the callee. The database may be implemented using either a server or a peer-to-peer (P2P) distributed database, or a combination of the two. Once the caller's client has retrieved the callee's IP address, it can then use the IP address to request establishment of a live voice and/or video stream between the caller and callee terminals via the Internet or other such packet-based network, thus establishing a call. An authentication procedure is typically also required, which may involve the user providing credentials via the client to be centrally authenticated by a server, and/or may involve the exchange of authentication certificates between the two or more users' client applications according to a P2P type authentication scheme.
Traditionally VoIP calls have been accessed using desktop or laptop computers, but increasingly nowadays the user tends to have a number of devices around the home or office that can potentially be used in VoIP calls or other such packet-based telephony. However, organizing or coordinating the use of these different devices is not necessarily straightforward for the user, or user may not even be aware when different devices are available to provide VoIP services or the like.